In order to prevent malfunctions in the increasing number of electronic components or in the power supply in the vehicle and related safety or comfort restrictions for the vehicle occupant, the demands on electrical components to keep the power draw from the vehicle onboard power supply system within defined limits are increasing. This applies particularly to electric motors in a motor vehicle brake installation, the high starting currents of which can result in severe loading of the entire vehicle onboard power supply system and in an intermittent drop in the supply voltage. This can in turn affect the functionality of other vehicle components and hence restrict safety and comfort for the vehicle occupants.
In this regard, DE 10 2006 006 149 A1, which is incorporated by reference, describes a method for actuating a displacement machine in a brake installation, which method has provision for a special starting method in order to avoid current spikes when the PWM-actuated drive of the displacement machine is started. To this end, a higher PWM frequency is used in the starting or primary phase than in what is known as the secondary phase, after the drive has started. Since the current spikes take place in a very short time range, the higher PWM frequency of the primary phase achieves improved tracking of the duty cycle, which achieves a reduction in these current spikes. In addition, the duty cycle is increased gradually in prescribed steps within the primary phase up to the maximum value of continuous actuation (duty cycle corresponds to 100%) in order to start the secondary phase subsequently. In addition, the special starting method can be activated, deactivated or modified on the basis of the vehicle travel state.
Systems according to the prior art no longer meet the present demands on the limitation of peak currents, particularly on the basis of the charge quantity provided by the voltage source and also the maximum slope of the current edges. It is not possible for the PWM actuation frequency to be increased arbitrarily, since the high computation workload of the microcontroller means that alteration of the duty cycle by the control software results in almost no further functions being able to be performed during this time. This clashes with a growing number of brake assistance functions, which make increasing use of resources. In addition, it is not possible to communicate with the hardware logic of the electric motor actuation at arbitrary speed. The communication interface, for example SPI, between the microcontroller on which the control software is implemented and the actuating hardware of the electric motor is a limiting factor for existing systems.
If, by way of example, commands from the software for changing the duty cycle are sent in the 1 ms pattern—even just 10 ms are usual at present—this is insufficient, when the electric motor is switched on, for limiting the peak currents and current edges in order to meet the present demands for avoiding malfunctions, since the rising edge of the current takes place in a time range of less than 1 ms. The current edge when the electric motor is switched off can also cause faults in the electronic components in the vehicle that need to be avoided on the basis of the demands.